1. Field of the Invention
This invention relates to method of, and flexible surface mold for, shaping heat softened sheets, and in particular, to method of, and mold having flexible straps and deformable end members for, biasing a heat softened glass sheet against an upper mold having a shaped surface.
2. Technical Considerations
Tempered and annealed shaped glass sheets used as windows in vehicles, such as automobiles, are required to meet stringent optical requirements, e.g. be free of optical defects that would tend to interfere with clear viewing through the window. Any distortion in the members that engage the heat-softened sheet during the shaping process is replicated in the major surface of the sheet and can result in the glass sheet having an optically deficient surface.
Commercial production of shaped glass sheets usually includes heating flat glass sheets to their softening temperature, shaping the heated sheets to the desired curvature, and then cooling the shaped glass sheets in a controlled manner to anneal, heat strengthen or temper the shaped glass sheet. In one glass shaping process, the glass sheets are serially conveyed through a tunnel type furnace to heat the sheets to their heat deformation temperature, and the heat softened sheets are thereafter conveyed into a shaping station where the sheets are positioned between a pair of vertically aligned upper and lower shaping molds. After shaping, the molds separate with the shaped glass sheet remaining engaged against the upper mold by vacuum. In one embodiment, e.g. disclosed in U.S. Pat. No. 4,830,650, a transferring tempering ring having an outline shape slightly inboard of its perimeter conforming to the desired curvature of the glass sheets moves below the upper mold. The vacuum is discontinued, and the glass sheet drops on the ring. The tempering ring conveys the shaped glass sheet through a cooling station to temper the sheet. In another embodiment, e.g. disclosed in U.S. Pat. No. 5,286,271, after the sheet is shaped, the lower mold is moved downward and the upper mold having the sheet held to the shaping surface by vacuum, moves to a transfer station where the shaped sheet is dropped onto contoured conveying rolls that move the sheet through a cooling station.
In the shaping processes discussed above, the heat softened sheet is pressed between a pair of upper and lower molds, each having a shaping surface. The upper mold is typically a full surface vacuum mold, and the lower mold can be a full surface press as disclosed in U.S. Pat. No. 4,662,925, a segmented mold as disclosed in U.S. Pat. No. 4,272,274 or a ring type mold as disclosed in U.S. Pat. No. 4,830,650. A limitation of the full surface lower mold and the segmented mold is that the molds can only be used to shape sheets to have the same contour. When the sheet contour of the upper mold changes, the full surface mold and the lower segmented mold have to be changed. A limitation of the lower ring mold is that the center portion of the sheet is not biased against the shaping surface of the upper mold.
As can be appreciated, it would advantageous to provide a lower shaping mold that engages and biases the center surface portions and outer surface portions of the sheet against the shaping surface of an upper mold and has a sheet engaging surface that is not limited to one shape or contour.